Coil system employing at least one highfrequency coil having a premagnetised rod-shaped core



Feb. 12, 1957 F. J. M. LATHOUWERS ,7

COIL SYSTEM EMPLOYING AT LEAST ONE HIGH-FREQUENCY COIL HAVING APREMAGNETISED ROD-SHAPED CORE Filed Dec. 12, 1952 N Ll N 7 3 S g a iFig. 4

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FRANCISCUS JOHANNES MARIA LATHOUWERS AGENT United States Patent COILSYSTEM EMPLOYIN G AT LEAST ONE HIGH- FREQUENCY COIL HAVING A PREMAGNET-ISED ROD-SHAPED CORE Franciscus Johannes Maria Lathouwers, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company,Hartford, Conn., as trustee Application December 12, 1952, Serial No.325,503

Claims priority, application Netherlands December 29, 1951 Claims. (Cl.336-110) The object of the invention is to provide a coil system ofcheap construction suitable for mass production and including at leastone high-frequency coil, the inductance of which is required to becapable of being adjusted once.

There are many coil constructions of this kind most of them based onvariation of the eifective permeability (reluctance) of the core as aresult of changing its effective section or its length or the size ofthe air-gap provided in the magnetic circuit, etc. The end to beachieved by all these constructions is to satisfy a number of opposedrequirements, namely low price, satisfactory coil quality, small sizeand a comparatively large adjusting range, each construction beingnaturally a compromise between the requirements.

The invention achieves a very favourable compromise by usingpremagnetisation for controlling the inductance, by means of a permanentmagnet made of a material of high electrical resistance arrangedadjacent at least one of the ends of the coil core to obtain adjustablepremagnetisation, a sliding member being provided to vary themagnetisation of the coil core produced by the magnet, the inductancechanging always in the same sense when displacing said member in onedirection within its sliding range. The sliding member may be the magnetitself.

The invention will be described with reference to the accompanyingdrawings, in which Figs. 1, 3 and 4 show various forms of a coil systemaccording to the invention.

Figs. 2 and 5 are graphs showing the operation of the coil illustratedin Fig. l.

The high-frequency coil shown in Fig. 1 comprises a bar-shaped core 1 offerromagnetic material on which a coil 3 is arranged. This coil isfixed, for example by means of wax or lacquer 2, inside an insulatingsleeve 5 provided with an internal screw thread. Adapted to be screwedin the sleeve 5 is a tubular axially magnetised permanent magnet 7 whichin some adjusting positions embraces the core 1 and effects adjustablemagnetisation of the core 1.

To prevent losses in the magnet 7-which is arranged in thehigh-frequency field of the coilit is made of high-resistance material(the specific resistance being at least 10 ohms-cm.) having lowhysteresis losses, for example ceramic material primarily composed ofnon-cubic crystals of polyoxides of iron and one of the metals barium,strontium, lead and if required calcium. The core 1 is preferably madeup of a cubic mixed crystal material of oxides of iron and at least twoother metals which combines low losses and high permeability(approximately 400) with comparatively low saturation, known asferrites.

Fig. 2 shows the inductance L in microhenries of a coil of the abovedescribed type as a function of the distance a in mms. between the coilwinding 3 and and the magnet 7. The length b of the part of the core 1projecting from the winding 3, which in some adjusting positions 72,781,496 Patented Feb. 12, 1957 ice is embraced by the magnet, was 6mms., the length 0 of the magnet 10 mms., and that of the winding also10 mms. The thickness of the core 1 was 1.6 mms.

As may be seen from this graph the inductance L has a comparatively highvalue in the position in which the magnet 7 engages the winding 3-inwhich a maximum saturation of the core may be expected-and initiallydecreases as the magnet moves further away from the winding 3. At adistance a of approximately 2.5 mms., a minimum value P is reached andthe inductance then increases to a point S, at which it substantiallyhas the value obtaining without premagnetisation being provided.

This behaviour is to be explained by assuming that at a distance a offrom 0 to 2.5 mms. the magnet 7 is shortcircuited to a greater or lessextent by the core 1 so that the external field of the magnet isconsiderably reduced. Beyond the point of minimum value P this effect isapparently reduced or even eliminated and inductance increases with thedistance a.

Both the range OP and the range P'S (P' and S are the abscissae of thepoints P and S) may be used to adjust the inductance of the coil, sinceshifting the magnet within these ranges in one determined directioncauses the inductance to change invariably in the same sense, that is tosay that it either increases or decreases. The entire area OS forexample is of no use for the said purpose. The area PS' yields thegreatest variation, a variation of approximately 46% being enabled withthe above described coil.

If the region P'S' is used the part (b) of the core projecting from thecoil winding 3 may be considerably shorter than the magnet. The core mayeven be short enough for the magnet never to embrace it, only the partRS' of the region PS being thus used. In this event the magnet may bebored for part of its length only or may not be bored at all but in thelatter case the variation of the curve of Fig. 2 is less regular. Fig. 3shows an alternative embodiment in which the coil winding 11 is wound inone layer and the magnet 13 has a comparatively wide bore enabling it tobe moved over the winding 11.

Fig. 4 shows a modification in which the short-circuiting of the magnetby the core 1 does not occur, but in this case the magnet 19 must bemagnetised radially, that is to say that for example the southpole (S)is produced on the internal wall of the bore and the northpole (N) onthe cylindrical external surface of the magnet.

The above described construction permits the manufacture of very smalland cheap coils of reasonably satis factory coil quality (w=angularfrequency, r=loss resistance). The latter property is largely due to thecoil winding being wound directly on the core, but that is to saywithout any intermediate space, the high permeability of the core beingthus fully utilised. Winding directly on the core is permitted on theone hand by the high electric resistance of the cubic core material, aseparate coil sleeve being unnecessary owing to this resistance, and onthe other by the fact that the inductance is adjusted without moving thecore in relation to the winding.

In Fig. 2 the curve 21 indicates the variation of the coil quality Q asa function of the control (distance a); as may be seen from this curvethe Q changes little by the control.

A further advantage of the above described constructionsin which thecore cannot move in relation to the windingconsists in that the coilwinding together with the core may be enclosed within a thin waterrepellent shroud, for example, of plastic.

As mentioned hereinbefore, it is highly important that the hysteresislosses in the magnet material should be low and in view thereof it isdesirable that the material should satisfy a particular requirementwhich will be explained with. reference to" Fig. 5.

This figure shows part of the magnetisation curve (BHcurve) of a usualform of magnet steel (curve I) and of one of a ceramic permanentmagnetic material for use in a coil system according to the invention(curve II). The intersections Br and Br with the Y-axis are theassociated values of the remanent induction and the intersections EH01and EH02 with the X-axis are the values of the coercive field strength.

In superposing a weak alternating field on the permanent field of thesteel magnet the latter field changes for example according to the loop41 in Fig. 5; the area of this loop is a measure of the hysteresislosses. The corresponding loop 43 of the curve II has shrunksubstantially to a line 43 which is coincident with the curve II andhence the hysteresis losses are very low.

The shape of the hysteresis loop and the correlated value of thehysteresis losses are found to be determined by the BrZBHc ratio It hasbeen found that the losses are sufiiciently low if the BI'IBHc ratio isless than 4. With ceramic permanent magnetic materials a value of thisratio may be achieved which slightly exceeds unity and the electricalresistance may be high. Such material is therefore particularly adaptedfor use in the device according to the invention.

What I claim is:

1. A high frequency coil system comprising a substantially bar-shapedferromagnetic core, a'coil winding on a portion of the length of saidcore whereby an end of the core projects a substantial distance from thecorresponding end of the coil, said coil and core being fixed relativeto one another, and a hollow, low-loss, highresistance permanent magnetcoaxially aligned with said coil and core at the ends thereof andproducing a magnetic field which premagnetizes said core, said magnetbeing axially movable over a predetermined range of distances relativeto said core, including distances at which said projecting end of thecore enters within said hollow magnet, for varying the premagnetizationof said core and thereby varying the inductance of said coil, said rangeof distances also having values at which the inductance of the coilalways changes in the same sense when the magnet is moved in onedirection.

2. A coil system as claimed in claim 1 wherein the magnet is magnetizedin the axial direction.

A coil system as claimed in claim 1 wherein the magnet is magnetized inthe radial direction.

4. A coil system as claimed in claim 1 wherein the core is constitutedof cubic mixed crystal ferrites, the magnet is primarily composed ofnon-cubic crystals of a composite oxide of iron and at least one of themetals barium, strontium and lead, and the coil is wound directly on thecore.

5, A coil system as set forth in claim 4 wherein the coil and core areenclosed in a water-repellent insulating sheath.

References Cited in the file of this patent UNITED STATES PATENTS2,000,378 Deisch May 7, 1935 2,413,201 Tillman Dec. 24, 1946 2,438,770Tillman Mar. 30, 1948 2,503,155 Harvey Apr. 4, 1950

